1. Field of the Invention
The present invention relates to methods of controlling ellipse interpolation by a NC(Numerical Control)-device and more particularly to such methods used in NC-devices which control the operation of a tool machine which requires complete-circle accuracy.
2. Description of the Related Art
Conventionally, a machine tool which performs 3-dimensional cutting, etc., realizes a stereooperation by causing driving mechanisms disposed for the corresponding X, Y and Z shafts to move cooperatively. Programmed numerical control devices are often used for such a multi-shaft cooperative control.
Such an NC-device analyzes respective movement command values for the shafts from a predetermined movement command program input externally, and sequentially outputs the respective analyzed command values into driving mechanisms for the corresponding shafts of the machine tool to cause the respective driving mechanisms to cooperate and thereby to cause the machine tool to automatically execute a desired operation set in the program.
In order to cause the NC-device to perform an operation such as cutting a complete circle internally or externally, simultaneous control of at least two axes is required. For example, in order to cause the NC-device to draw a complete circle having a radius of r with the coordinates (x.sub.O, y.sub.O) as the center in an X-Y plane, an X axis coordinates x and a Y axis coordinate y satisfy the equation: (x-x.sub.O).sup.2 =(y-y.sub.o.sup.2 =r.sup.2 and the respective command values for the shafts are simultaneously controlled in accordance to an angle .theta. in the equation of x=(r cos .theta.+x.sub.O) and y=(r sin .theta.+y.sub.O).
Such a simultaneous control method of the two axes will involve an error and all the conventional control methods of a complete-circle operation can almost be said to be a control method of an ellipse operation. The error factors for causing such an ellipse operation involve the pitch between any adjacent shafts, a thermal deviation of each of the shafts, a thermal deviation of a main shaft head, a change in the machine tool body and aging of the control device and the machine tool body.
Basic measures are taken to increase the static accuracy and rigidity of each of the shafts. In order to cope with the thermal deviations, heat sources are suppressed or eliminated. In addition, a correction corresponding to each of the error factors is performed; e.g. a pitch error correction, a gradient correction, a thermal deviation correction and the like.
The measures against, or correction to, the error causing the ellipse are taken or performed for each of the factors in equation. Therefore, in order to increase the accuracy with which complete-circle-oriented machining is performed by a system comprising the NC-device and the machine tool, special techniques must be understood corresponding to the characteristic and control type of the machine tool itself. Therefore, it is very difficult or impossible for a general user to cope with these problems.
For example, in order to increase the static accuracy, a skill is required for measuring and adjusting the squareness of the angle between any adjacent shafts and the straightness of the respective shafts the above and special measuring instruments are required to execute. Therefore, measurements also, in order to suppress or eliminate the heat source, measurements must be taken against many heat sources, such as the main shaft head, ball screws and hydraulic units. Therefore, there are problems such as a complexity change of structure and an immense increase in the additional cost. In addition, these measurements have not perfectly eliminating these errors.
The correction of pitch errors is made for expansion and contraction of the respective shafts. The gradient correction is made for the squareness between adjacent shafts and the straightness of each of the shafts. However, such corrections are temporary and require a skilled person's technique and special measuring instruments. The correction of thermal deviations include correcting those deviations on the basis of the beforehand measured effects of heating due to various causes; this is effective. However, the correction to the thermal deviations is not effective for prevention of the generation of errors involved in the machine.
The mechanical accuracy of the control device and the machine tool occur due to aging. The correction in the control device is to set optimal values for the conditions under which the correction is made since there are no effective measures against aging.